Currently, laser beam having wavelength of an ultraviolet region has been used for microfabrication of metals, reading devices of optical discs, and the like.
Conventionally, such laser beam having wavelength of the ultraviolet region has been generated by using gas laser such as excimer laser.
However, gas laser has problems of low operation efficiency and short lifetime, and also has a problem that F2 gas, for example, which is used as operating gas has toxicity. Further, a problem of high operating voltage has been also pointed out.
For these reasons, technology has been proposed to generate laser beam having wavelength in the ultraviolet region by solid-state laser using a non-linear optical crystal instead of gas laser.
As such technology, a technology is known where non-linear wavelength conversion is performed to laser beam having the wavelength of 532 nm, which is the second harmonic of the Nd:YAG laser, into laser beam in an ultraviolet region with the wavelength of 266 nm by using a cesium-lithium-borate crystal (CLBO crystal).
Meanwhile, the inventor of the present invention discovered that the CLBO crystal, because it is uneven and the crystal has irregularity, had a problem of reduction of output in a very short time by the generation of damage on the crystal caused by ultraviolet ray, which is created by the non-linear wavelength conversion, depending on a region to which laser beam is irradiated for non-linear wavelength conversion. Further, the inventor also discovered a problem that it was impossible to obtain a same output constantly because of the unevenness of crystal even if the region to which the laser beam having the wavelength of 532 nm was changed by simply moving the crystal when the output reduced.
The present invention has been made in view of the above-described problems, and its object is to provide a method of evaluating a non-linear optical crystal and a device therefor, and a wavelength conversion method and a device therefor, which are capable of evaluating the non-linear optical crystal in advance before the non-linear optical crystal is actually used as a wavelength conversion element in order to maintain high conversion efficiency and use the non-linear optical crystal stably for a long period of time without reducing output in the case where laser beam wavelength is converted by using any of various kinds of non-linear optical crystal for non-linear wavelength conversion such as a CLBO crystal.